The present invention relates to a method of coating a workpiece by spraying a coating material on the workpiece from a spray gun mounted on a coating device such as a robot or a reciprocator while moving the spray gun along a predetermined locus.
In a coating system as shown in FIG. 4 by way of example, a coating device (not shown) provided with a spray gun is located in opposition to a lateral side of a conveyor 20 for conveying a plurality of workpieces 21 and 22 in a direction depicted by an arrow shown in FIG. 4. The spray gun is reciprocatively moved substantially perpendicular to the moving direction of the conveyor 20, and is controlled to become on and off with a suitable timing, thereby carrying out electrostatic coating of the workpieces 21 and 22. A locus of relative movement of the spray gun with respect to the workpieces 21 and 22 is shown by a broken line in FIG. 4.
In the coating system as mentioned above, when the spray gun is merely on/off controlled in accordance with a shape of each of the workpieces 21 and 22, a coating film becomes thick at an end portion of each of the workpieces 21 and 22 by the influence of static electricity, or a coating film becomes thin at an upper end portion and thick at a lower end portion of each of the workpieces 21 and 22 by the action of air flowing from an upper area to a lower area in a spray booth.
To solve this problem, an on-region of the spray gun with respect to each of the workpieces 21 and 22 is that surrounded by a two-dot chain line shown in FIG. 4.
However, in the conventional coating system as mentioned above, the on-region of the spray gun with respect to the lateral edge of each workpiece slightly shifts due to the; relative movement of the spray gun with respect to the workpiece as shown in FIGS. 5A and 5B, so that the spray gun becomes on or off according to a pass thereof.
Accordingly, when the spray gun becomes off, a coating film becomes thin at a side end portion of the workpiece, while when the spray gun becomes on, the coating film becomes thick at the side end portion of the workpiece.
As a result, the coating film becomes nonuniform in thickness to reduce a coating quality.